Acoustic wave sensors have gained significant importance in modern life, either as part of a voice transmission device or of a photoacoustic sensor employed in a gas analyzer for analyzing gases such as ambient air.
Commonly used acoustic wave sensors are configured as capacitive sensors having two membranes spaced apart from each other and defining a capacitor therebetween. One of the membranes is fixed and the respective other one is displaceable by the acoustic waves to be detected. A displacement of the displaceable membrane is indicative of characteristics of the acoustic waves to be detected and induces a change of the capacitance of the capacitor that can be detected by a suitable read-out circuit providing an electric signal indicative of characteristics of the acoustic waves to be detected such as of the acoustic pressure.
Although capacitive acoustic wave sensors achieve high sensitivities, they have several drawbacks mainly arising from their complex structure.
The drawbacks of commonly used capacitive acoustic wave sensors regarding their complex structure can be overcome by piezoelectric acoustic wave sensors. This kind of sensors employ a thin film made of a piezoelectric material deflectable by acoustic waves to be detected. A deflection of the piezoelectric film induces a voltage in the piezoelectric film that can be detected by a suitable read-out circuit providing an electric signal indicative of characteristics of the acoustic waves to be detected.
Although piezoelectric acoustic wave sensors may be provided with a simpler structure as compared to capacitive acoustic wave sensors, they suffer from lower sensitivities as compared to capacitive acoustic wave sensors.